1. Field of the Invention
The invention relates to devices for preventing thermal and mechanical stresses in internal combustion engines. In particular, this invention relates to an automatic control device associated with the governor of an internal combustion engine for regulating the fuel pumps during engine overload conditions.
2. Description of the Prior Art
The main diesel engine of all ships that have direct rotation from the engine to constant pitch propellers perform according to the propeller characteristics. Propeller characteristics depend upon effective parameters of working conditions derived to the propeller, or propeller torque, and other parameters of working conditions derived from the number of revolutions of the diesel engine when it is working to the propeller. Working conditions vary according to the ambient conditions at any given time--wind, sea conditions, keel draugh, towing or load, physical condition of the propeller and ship's hull, depth of water, etc.
In the operation of diesel engines with constant pitch propellers, the two most important speed considerations are those under full speed, and those under loaded or towing conditions. Full speed is necessarily variable to accommodate a wide range of loaded conditions, and the power requirements vary accordingly.
When a ship enters shallow waters, the resistance by the ship's hull increases, and the power expenditure required to overcome the wave action and water resistance increases. With an increase in power requirement, there is a resultant increase in combustion pressure, exhaust gas temperature, and mean indicator pressure. Exceeding the manufacturer's limits of these characteristics causes the engine to undergo both thermal and mechanical stress. At the same time, considerable increase in fuel consumption is required with a decrease in forward speed. To overcome the stress situation, it has been necessary to decrease the fuel supply manually from the control room or the bridge.
Under towing or loaded condition, which are the principal heavy working requirements of the engine, the operator determines the optimum control necessary to prevent the placing of stress on the engine. This is accomplished by constant observance of exhaust gas temperature or power indicator gauges. Due to continuing changes in ambient conditions encountered by the ship, the control room operator may choose to delay taking corrective action, since conditions may reverse at any time. However, engine operation under overload conditions for a period of 10 to 20 minutes can result in additional fuel consumption of 25%-35% for that period with no increase in speed of travel. Moreover, extensive damage to the engine may result from stress.
The majority of diesel engines are equipped with variable speed governors which control the operating speed of the engine. By the addition of a control device to the variable speed governor, it is possible to automatically protect the engine from thermal and mechanical stress. By automatically preventing the engine from overloading, it is feasible to realize a savings of 25 to 35% in fuel consumption during overload periods. Further, a saving is also realized in prolonging the normal engine life. The entire concept of an automatic control device is to keep the engine performance within the limits of the manufacturer's operating specifications under all load conditions whether constant or varying.
An automatic control preferably should be designed for bypass by the operator during any period of emergency for the ship where engine stress would not be a consideration.
Prior attempts to protect against stresses on marine diesel engines have generally fallen into three categories. A first method is a manual control method whereby the exhaust gas temperature is continuously monitored as an indication of the load on the engine. When the exhaust temperature goes above a particular maximum for a given engine, e.g., 375.degree. C., the control room operator manually takes corrective action by reducing fuel input to the engine. This method has an inherent limitation in that the exhaust temperature must be constantly monitored and the corrective action must be taken according to subjective determinations made by the operator.
A second method utilizes a so-called position feedback system including thermistors which measure the engine exhaust temperature. The signals from the thermistors, after amplification, are utilized to control the governor thereby reducing fuel consumption and engine speed. A primary disadvantage of this method is that it reduces fuel consumption only in response to increased temperature; therefore, it is cyclic in nature so that the engine must be continuously heating up and cooling down in order for the system to work. Such systems have been provided with memory elements to smooth out the cyclic nature of operation, but they have proved extremely costly and ineffective in operation.
A third approach has been a system utilizing ultrasonic sounders to continuously monitor the depth of the water in which the ship is operating. The depth signal is utilized to program the governor. Obviously this "self-governor" method is limited to ship applications and is operable to sense only overload conditions caused by shallow waters. Furthermore, the art has yet to provide ultrasonic sounders which are accurate in shallow waters due to the interference from air bubbles which are found in shallow water.
In addition to the need for an automatic means for correcting for the loading conditions on a marine engine, it is equally important that the fuel economy of the engine be maintained at its most economical setting and to maintain as constant a ship speed as possible. That is, an engine that is experiencing heavy loading conditions and the ship's speed is reduced, the power consumption will go up without an increase in the speed. This added power input is reflected as increased heat in the engine, and is a direct result of an increase in the fuel input. It is a characteristic of ships that approximately the same ship speed, resulting from loading conditions, can be maintained at a reduced fuel consumption rate and at a lower engine speed. This reduced fuel consumption can be substantial, approaching 25 to 35% fuel savings.
Under loading conditions, an engine that attempts to work harder to maintain its normal engine speed setting, merely tends to increase the loading thereon. For example, a ship which is in shallow water and experiencing a travel speed reduction due to greater drag, may attempt to increase the fuel input to the engine to regain the lost travel speed. Unfortunately, the harder the engine tries to push the ship through the water, the greater the drag or resistance exerted by the water on the ship. As a result, the loading increases and causes a further decrease in the travel speed.
Thus, the present state of the art shows that there is an acute need for a reliable and simple automatic device to protect against thermal and mechanical stress on internal combustion engines, especially marine diesel engines, and to operate the engines under loading conditions at the most economical fuel consumption rate possible.